Mr.DXt

okay the story is i have a automatic y8 complete long block that i bought and i swap my 5psd tranny over onto it so i can drop into my car (1996 civic dx but running the ex vtec engine harness). After its been dropped in the car starts fine without any issue but then it idles weird it jumps up and down from 1500-2000 rpm. Ive change the iacv to a working condition one and it still have that same issue. The thing was when i got the long block the intake manifold was dismounted and the gasket was all bad so than i replace it with a autozone gasket...could that gasket be the cause of idle surge? or if anyone knows what might be the cause please let me know? thanks is anyone could help...

EK2.0

Check all your vacuum lines, it sounds like a classic honda idle leak, make sure all your gasket seals and vacuum line connections are good. Especially that intake mani. gasket!

Mr.DXt

okay thanks no vacuum leaks so far just order a gasket from honda so ill be changing that tomorrow well see how that goes.. does anyone know whats the worst case scenario that could cause this issue?

EE_Chris

Where to start...

A stock 96 Civic Dx 5-spd uses a throttle body mounted 3-wire IACV...which is controlled by a "3-wire IACV" ECU. A stock Civic Ex auto uses the same IACV setup, however a Civic Ex 5-spd uses a 2-wire IACV (and appropriate ECU). To be clear, ECU's meant to control a 2-wire IACV will NOT control a 3-wire IACV and vice-versa. Then in 99 & 00, Honda decided to add a pseudo-FITV system to the Ex which included an intake manifold that has a small hole above each intake runner (which lead internally to a barb/nipple sitting on top of the mating flange to the head).

So, what parts are you actually using:

ECU (auto/5-spd and year?)
IACV (2/3 wire?)
Throttle body (does it have an IACV attached to it?)
Intake manifold (auto/5-spd, does it have an IACV attached to it, and year?)
Wiring harness (you mentioned Ex VTEC - auto or 5-spd?)

Mr.DXt

ex manual ecu p2p?
IACV 2 wire
stock automatic throttle body (not sure on the iacv on that how do i check?)
Intake manifold automatic one but switch ex manual IACV on
ex manual engine harness

EE_Chris

Alright - the stock automatic Y8 throttle body has a 3-wire IACV attached to it. 3-wire IACV's don't close down when they are unplugged (unlike the 2-wire variety)...which means that's potentially a vacuum leak.

So you're using the auto-Y8 intake manifold that you drilled/tapped to let it accept a 2-wire IACV? That's fine if you did...but is there a barb/nipple on the flange that mates to the head? Because if it's a 99-00 Y8 manifold and you haven't sealed those ports off, that will cause a vacuum leak too.